The present invention relates to a gasket, in particular, to a gasket suitable for use in sealing a joint between fluid-passage blocks in which through-holes are provided to form a passage for the supply of a highly pure fluid or liquid to an apparatus used in the production of semiconductor devices.
These fluid-passage blocks are generally made from stainless-steel, and gaskets used to seal joints formed between respective blocks may be made from a metal having super-elastic properties. The use of a super-elastic metal in forming a gasket has the advantage that upon release of a securing force used to tighten the gasket in forming a seal, the gasket reverts to its original shape, whereby it can be reused; and, in addition, such gaskets are capable of accommodating deformation of the fluid-passage blocks caused by exterior forces to which sealed blocks may be subject. Japanese Published Utility Model Application 6-20946 discloses such a gasket. Further, Japanese Laid-Open Application 10-311450 discloses an example of assembled fluid-passage blocks.
FIG. 11 shows common cross-sectional forms of conventional gaskets, These are, an O type, a C type, and a C-O type (C type with a O shaped spring). The O type gasket is the most commonly employed and is able to retain its function under a wide range of environmental stresses, ranging from super-high pressure to super-high vacuum. The C type gasket is advantageous in that by using a relatively weak sealing force as compared with the O-type gasket, an effective seal can be attained under super-high pressure. The C-O type gasket has a high capacity to revert to its original shape upon the release of a sealing force applied thereto; and such a gasket is able to maintain appropriate sealing under cyclic temperature and/or pressure changes. Another type of gasket which has recently come into use exhibits plastic deformation when subject to a sealing force and thus is able to conform with a high degree of accuracy to the shape of the joint between blocks to be sealed.
FIG. 12 is a cross-sectional view of a conventional fluid-passage block which is provided with a gasket receiving recess having a depth xe2x80x9cwxe2x80x9d. FIG. 13 is a cross-sectional view of a part of the left halves (as viewed in the figure) of the fluid-passage blocks 3 and 4 which have been assembled with a gasket 100 interposed therebetween.
FIGS. 14 and 15 are diagrams showing characteristics of gasket materials. FIG. 14 is a stress-strain diagram relating to a metal which is commonly used to make gaskets, and FIG. 15 is a stress-strain diagram relating to a super-elastic metal as mentioned above.
It is to be noted that conventional gaskets suffer from the following defects.
First, in gaskets made from a metal such as stainless steel or nickel, both of which exhibit a relatively narrow range of elastic deformation under the imposition of a sealing force, plastic deformation is liable to occur if an excessive force is applied, thus compromising the sealing properties of the gasket.
Second, even if the gasket is not subject to the plastic deformation, once the gasket is temporarily detached from the surfaces of fluid-passage blocks, the gasket might become ineffective in sealing the surfaces of fluid-passage blocks. Consequently, it is required that the gasket is fastened with a large fastening force.
Third, in a gasket made from a metal such as stainless steel or nickel, frequent mounting and dismounting of the gasket will result in molecular changes in the metal of the gasket in relation to surfaces to be sealed, resulting in an ineffective seal.
Fourth, since gasket receiving recesses formed in surfaces of fluid-passage blocks may have a tolerance of, for example, xc2x130 microns with a consequent cumulative variation between recessed surfaces of xc2x160 microns, and as it is also necessary to compensate for any possible variation in size of the gasket per se, a relatively large amount of portion of the gasket is required to be compressed or deformed; for example, such a compressible amount is generally required to be in the order of about 200-400 microns.
Fifth, since most conventional gaskets exhibit plastic deformation to some degree upon being subject to a sealing force, the original shape of the gasket is liable to be lost, making repeated use impracticable. In extreme cases, gaskets made from certain kinds of metals may be able to be used once only.
Sixth, even where a gasket is made from a super-elastic metal, there is a danger that plastic deformation will occur due to an excessive fastening force which might be mistakenly imposed thereon, thus resulting in a deterioration of restorative properties of the gasket, and a consequent decline in its sealing properties.
Seventh, in a conventional gasket, if a surface of a fluid-passage block contains a flaw, or a surface of the gasket to be engaged with the surface to be sealed contains a flaw, it is likely that the gasket will be unable to provide an effective seal.
Lastly, surfaces of gaskets and fluid-passage blocks to be sealed by gaskets are required to be finely machined, as any irregularity in the surfaces will prevent a proper seal from being formed. Such fine machining is both costly and time-consuming.
The present invention has been made in the light of the above-noted problems to which the various prior art gaskets are subject
According to the present invention, there is provided a gasket comprising a sealing portion and a deformation stopper portion. When the gasket is placed between adjacent surfaces of two objects, the sealing portion is engaged by the adjacent surfaces. The deformation stopper portion is engaged by the adjacent surfaces of the objects when the sealing portion has been deformed by a predetermined amount by the adjacent surfaces, thereby preventing excessive deformation of the sealing portion.
In one embodiment, the deformation stopper portion has opposite sides facing adjacent faces of the objects and the sealing portion also has opposite sides facing the adjacent faces of the objects, with the opposite sides of the sealing portion having compressible portions extending towards and positioned close to the adjacent surfaces of the objects relative to the opposite sides of the deformation stopper portion.
The sealing portion and the deformation stopper portion may be annular and the deformation stopper may be positioned radially outside the sealing portion. The deformation stopper portion has a radially outer surface, with a gasket plate engaging recess being provided in the radially outer surface so that, upon installation, the gasket is supported by a gasket support plate in such a manner that the gasket extends through a through-hole provided in the support plate, with the recess being engaged by projections formed along the peripheral edge of the hole.
The gasket may be made from a variety of metals, including super-elastic metals. The gasket metal may be harder than the metal of the objects having surfaces to be sealed.